(618c) Catalytic Hydrogenation of CO2 to Methanol Enabled By the Metal-Lewis Acid Interfaces in Metal-Organic Frameworks (MOFs) Uio-66

CO₂-to-methanol conversion is an important process as it offers a sustainable path to reduce the greenhouse gas CO₂ to industrially valuable chemicals. Traditionally, this reaction is catalyzed by the bifunctional Cu/ZrO₂, but the precise control of catalyst structure is still a challenge. The emergence of new Metal-Organic-Framework (MOF) materials offers unique platforms to tune the spatial locations of different active sites and their environments, providing a novel approach to control the Cu/support interface which is thought to drive CO₂ hydrogenation. In this work, we have demonstrated that depositing Cu clusters into UiO-66 frameworks enhances methanol production significantly, owing to the increased concentration of the interfacial Cu-Zr sites. First-principles density functional theory (DFT) calculations are carried out herein to gain insights into the catalyst structures and reaction mechanisms.

Our DFT calculations show that the interface Zr-Cu is the active site as the Lewis acid Zr⁴⁺ ion activates and leads to the strong adsorption of CO₂. The computed adsorption energy of CO₂ (ΔH_ads = 0.92 eV) agrees well with the measured adsorption energy. The exposed Cu surface easily dissociates H₂ to provide hydrogen sources to hydrogenate CO₂. From extensive mechanistic studies by DFT simulations, we propose that CO₂ preferentially undergoes stepwise hydrogenations via formate to methoxy and methanol, in which the formate hydrogenation step is thought to be kinetically relevant. The unselective CO is more likely produced via the direct CO₂ dissociation. Lastly, we have expanded this study to other isostructural compounds of UiO-66 made of Hf, Ce, and Th nodes. The results show that the Hf-UiO-66 has similar activity as the original UiO-66, while Ce and Th variants are less active towards CO₂ reduction. We attribute this behavior to the similar Lewis acidity, probed by NH₃ adsorption, between Zr⁴⁺ and Hf⁴⁺ ions, which are stronger than Ce⁴⁺ and Th⁴⁺.